The multicarrier modulation scheme is known including, for example, the discrete multi-tone (DMT) modulation scheme that modulates a plurality of carrier signals having different frequencies using an information signal to thereby frequency-multiplex a plurality of information signals. The DMT modulation scheme divides a use band into subchannels to thereby modulate frequency channels (subcarriers) through quadrature amplitude modulation (QAM). The DMT modulation scheme allows the number of bits used to transmit the information signals and assigned to each frequency to be varied according to a signal-to-noise (S/N) ratio. The DMT modulation scheme thus can transmit a large volume of information in a manner of making effective use of the use frequency.
The multicarrier modulation scheme, however, involves nonlinear distortion such as harmonic distortion occurring in each carrier signal to be modulated with the information signal and thus a need exists for a technique that reduces the nonlinear distortion in each carrier signal.
A known technique, although not in the field of the multicarrier modulation scheme, reduces the nonlinear distortion of an optical signal by superimposing a low-frequency signal over the optical signal. Incorporating this technique, however, requires a circuit used for superimposing the low-frequency signal over each carrier signal, thus resulting in a complicated circuit configuration.